In general, display data for a display device such as an LCD (Liquid Crystal Display) or a CRT (Cathode Ray Tube) is stored in a DRAM which is used as a main storage device or a dedicated storage device for display data. An LCD controller sends a request to a DRAM or a DRAM controller when display data is necessary, and reads out display data from the DRAM to display images and characters on an LCD according to the display data.
However, when it is necessary to allow images to be displayed at a low power consumption, a system which reads out display data from a DRAM in response to a request from the LCD controller is wasteful in terms of power consumption. That is because the operation to open a requested page of the DRAM in response to a request from the LCD controller and to close (precharge) the requested page after necessary display data has been read out is frequently performed. Here, the term “page” refers to the data unit size of a DRAM, which is to be activated by raw activation.
On the other hand, a technique is envisioned in which in order to reduce the number of times to open/close the page of DRAM thereby reducing power consumption, the page is kept in an open state by not closing it. For example, according to the technology disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 10-105367), access is controlled in a state in which a page of a bank of DRAM, which is currently being accessed, is kept open.
Further, in the technology disclosed in Patent Document 1, frame data is set so as to be contained in a page of image memory, and frame data of each of a pair of adjacent rectangular regions is adapted to correspond to an arbitrary bank respectively. Then, the page of any bank which will be accessed in the future is opened in advance so that any bank is successively accessible even when the bank to be accessed is changed.
As described above, in Patent Document 1, by utilizing the parallelism of the banks to be arbitrarily accessed, the rate of data transfer is increased. However, since the page of each bank is kept open and therefore since the DRAM needs to be always kept active, it is not possible to switch the DRAM into another mode, for example, into a power saving mode such as a self-refresh mode, and therefore power consumption is still large.
In order to solve the above described problems, a method is envisioned wherein all of the display data of the LCD is stored in a buffer, which is implemented in the same semiconductor integrated circuit device as the LCD controller and which is made up of an SRAM and a flip-flop other than a DRAM. By doing so, there will be no need to access the DRAM at all. Therefore, it is possible to cause the DRAM to shift into a power saving mode thereby reducing power consumption.
However, the display size of an LCD in recent times has increased and it is not realistic any more to store all the display data in a buffer. For example, in the case of a VGA size LCD having an 18-bit color display, the size of display data exceeds 1 Megabytes. If a buffer for storing all of the aforementioned data is implemented in the same semiconductor integrated circuit device as the LCD controller, the area of the circuit will increase, which is disadvantageous with respect to the manufacturing cost of the semiconductor integrated circuit device. Therefore, if hereafter the display size of the LCD increases, it is necessary to read out the display data from the DRAM, and the problem of power consumption will arise again.
Further, when an image to be displayed is smaller than the display size of an LCD and when the image is enlarged to be displayed on the LCD, it is necessary to read out the display data of the same horizontal line of the LCD for multiple times. Therefore, it is necessary to repeatedly read out the data of the same horizontal line from the DRAM, which will also increase power consumption.
Patent Document 1: Japanese Patent Laid-Open No. 10-105367